PARTHENOGENESIS IN THE ANIMAL KINGDOM. 2o7 



individuals by gemmatiou from buds, wbicli take the place of male 

 aud female organs of generation, and thus represent their nurse. 



On the coasts of the North and Baltic Seas immense swarms of clear, 

 watery, bell-shaped creatures may be perceived in summer, swimming 

 slowly around below the calm surface of the water, with their convex 

 surface upward and their concave downward. These are the Aurella 

 auriia^ L., a species of acraspedote, or unfringed medusa, some of which 

 are male and some female, as is tlie case in all medusa'. The sexual 

 organs are ruffle-like folds on the inner skin of four bags or folds in the 

 gastrical cavity, which open outward at the bottom of the stalk. By 

 simple ciliary motion the seed of the male passes into the bags of the 

 female and fecundates the eggs. These then pass out into the folds of 

 the tentacles, where they are develoj^ed to embryos, which are provided 

 with a very tender covering of cilia, and move about freely in the water 

 like infusoria. This phase of evolution was formerly considered as a 

 separate species, called planula. Soon, however, the cilia falls off, and 

 the animalcule, thus deprived of its locomotive organs, sinks to the 

 bottom, attaclies itself to firm objects, and grows longer. In the free 

 end a cavity soon appears, which gradually increases and is developed 

 into a mouth, from which wart-like excrescences or i>apilUe shoot out 

 and are afterward converted into tentacles. The animal has now the 

 appearance of a polypus; aud it was, indeed, formerly so considered, 

 and called hydra tuba. After some time — perhaps months — a circular 

 depression is seen just below the crown of tentacles, followed by others 

 behind it. These depressions become deeper aud deeper, aud short 

 projections appear in their edges, which afterward also develop into 

 tentacles. The whole now bears a distant resemblance to the so-called 

 strohilaj or fir-cone, or to a set of flat cups resting on a columnar foot, 

 the polypus. The separate divisions of the strobila are the origin of 

 the future medusa. They develop more and more, one" after another, 

 separate from their pedestal, and afterwards attain their permanent 

 form, size, and maturity. They now turn the convex surface by which 

 they were attached, upward, while the mouth, which was before turned 

 up, now points downward. In the aurelia there is, therefore, an inter- 

 mediate or nurse generation during the polypus stage, in which the 

 animal is multiplied in an agamic way by gemmation aud fission. 

 Each of the individuals so produced is again developed into a sexual 

 medusa. 



In medusie of lower organization belonging to the hydroids, which 

 Gegenbauer has called craspedote, because their disk is provided with a 

 velum, a similar kind of alternate generation takes place, with the ex- 

 ception, however, that the polypoid nurse reaches a much more advanced 

 stage of independent development after leaving the ovum. It grows to 

 a stalk of considerable size, and puts forth numerous polypus-buds. It 

 is only when the colony has attained a high degree of development that 



